Microwave ablation (MWA) is a form of thermal ablation used in interventional radiology to treat cancer. MWA is known for its quicker patient recovery and fewer complications and can serve as an alternative when surgical resection cannot be applied. MWA uses electromagnetic waves in the microwave energy spectrum (300 megahertz to 300 gigahertz) to produce tissue-heating effects, i.e., to heat tumors to cytotoxic temperatures. MWA is generally used for minimally invasive treatment and/or palliation of solid tumors in patients. MWA offers several advantages over other ablation technologies such as radiofrequency (RF) and cryoablation including higher temperatures than RF, larger ablation zone volumes, shorter ablation times, and better ablation performance near arteries, which act as heat sinks. Selective delivery of energy to the prescribed tissue volume (i.e. the tumor and its margins) is achieved by means of interstitial placement of a microwave antenna directly into the tumor. Current MWA technology may be employed either laparoscopically or percutaneously, and thus, is considered to be minimally invasive. However, the extent to which MWA is minimally invasive depends on a length and a diameter of the interstitial microwave antenna. In general, a MWA antenna is expected to have a small overall diameter, a low reflection coefficient, and localized specific absorption rate (SAR) and heating patterns.
Most MWA antennas employ coaxial cables as their feed lines. Coaxial cable, however, is an unbalanced structure and current can flow on the outer surface of its outer conductor if the cable is not properly terminated. If not properly suppressed, this current can lead to unwanted heating of the healthy tissue along the insertion path of the antenna along the coaxial cable. This current can also cause the reflection coefficient of the antenna to be dependent on the insertion depth into the tissue.
Coaxial baluns have been the most ubiquitous solution for overcoming problems associated with the unbalanced currents flowing on the outer surface of the outer conductor of the coaxial cables. A coaxial balun is generally implemented by encompassing the outer conductor of the coaxial cable with another conducting cylinder. The inner surface of this extra cylinder and the outer surface of the outer conductor of the coaxial cable form a transmission line. The length of the balun and its termination are chosen such that a very large impedance is seen at the tip of the balun by the unbalanced currents. This high impedance prevents flow of unbalanced currents beyond the tip of the balun and greatly reduces the level of unwanted heating along the shaft of the antenna. While coaxial baluns help the antenna in providing a fairly localized SAR pattern, they increase the overall diameter and, as a result, the invasiveness of the MWA antenna.
A sector of the outer conductor of the coaxial cable along with its inner conductor may be extended beyond the feed point. These two extended conductors act as two arms of a dipole antenna, where each arm of the dipole may be a quarter of a wavelength long. Since currents flowing on the arms of this dipole antenna oppose each other, a very low feed point impedance is achieved. This very low feed point impedance almost shorts the current at the feed point and prevents its flow on the outer surface of the outer conductor. However, while unwanted current is effectively suppressed, the impedance match is poor requiring an impedance matching structure.